You can turn on cruise control by pressing the C key (which can be remapped). Accelerate forward or in reverse to the desired speed and press the C key. The locomotive will now try to maintain this speed until you press the C key again to deactivate Cruise Control.
While driving in Cruise Control you can slow down the locomotive to pickup survivors by pressing the opposite direction key and this will decelerate the locomotive. When you let go off the key Cruise Control will take over and increase the locomotive speed back to the target speed.
Example:
I've accelerated my locomotive to 4.3 m/s by pressing the left mouse button. I then press the C key to have the locomotive maintain this speed. I can let go off the left mouse button and the locomotive will maintain speed. Zombie hordes will slow down the locomotive but once they are defeated the locomotive will accelerate back to 4.3 m/s. If I'm passing a survivor I can slow the locomotive without breaking cruise by pressing the right mouse button. This will slow down the locomotive. Once the survivor is onboard I let go off the right mouse button and the locomotive will accelerate back to 4.3 m/s.
Time to cruise to victory Choo Choo Engineers!